Both the magnitudes and speciation of phosphorus (P) released from the watersheds of two subtropical deep reservoirs in Taiwan, Tehchi and Feitsui Reservoirs were investigated. The dissolved organic P, together with the dissolved reactive P and the particle bound reactive P were categorized as the bioavailable P (BAP) in this study. Field data were used to identify the most important species of P thatfacilitated the growth of the dominating phytoplankton. Dissolved organic P supposedly from the decomposition of rot off in the forested area (mainly the Taiwan Red Pine), accounting for 30 % of the total P, was the leading type of background P from the watershed and served for the prosperity of the dominant algal species, Peridinium spp., in Tehchi Reservoir. Reactive P accounts for 62% of the total P in the runoff in the catchment of the oligo/mesotrophic reservoir, Feitsui Reservoir, with noncalcareous geology. The reactive P is the readily source for the growth of the dominating Cyanobacteria (mainly the Microcystis spp. and Aphanocapsa delicatissima) and many other species of algal in the surface water. The biodiversity of the phytoplankton ecosystem was suggested correlated with the geochemical P cycle in this catchment.